1. Field of the Invention
The invention broadly relates to a finishing process for elastomers. More particularly, the invention pertains to an apparatus for recovering dry polymer in porous crumb form by means of pressure extrusion at relatively low temperatures and pressures, and wherein the polymer extrudate shape is controlled. Furthermore, the invention provides means for conveying polymer extrudate in a manner which reduces crumb breakdown.
Conventional polymer recovery processes in elastomeric finishing operations involve drying elastomers, such as ethylene propylene rubber and butyl and halobutyl rubbers, in an extruder. The polymer, containing a certain percentage of water, typically is fed to an extruder comprising a rotating feed screw which pressurizes and heat plastifies the material upstream of a die means. The die means, which is mounted at the discharge end of the screw press, generally includes a number of die orifices through which the material is forced. As the material is exploded out through the die orifices, the water is flashed off and steam is released. A cutter disposed adjacent the discharge end of the die means cuts the extruded material into particles.
In order to remove water from the polymer, it is necessary to generate high temperatures and pressures at the extruder die. In order to obtain the requisite high temperature and pressure, a die plate with small round die orifices is typically utilized in conventional systems. The high temperature and pressure associated with such systems results in the proliferation of fines and reduces process line capacity for the finishing operation. Moreover, the conventional cutter typically possesses a plurality of radial blades rotating at high speeds. The design features of typical cutters, in conjunction with their high speed, tends to cut the extruded crumb, thereby contributing further to the production of fines, which are generally considered to be particles smaller than 2 mm in size. The high cutter speeds which conventional cutters must assume in order to avoid plugging contributes to higher fines generation.
An additional detriment associated with conventional extruder drying processes involves the conveying of the particulate material away from the cutter. In some conventional finishing processes, a pneumatic conveying system is employed to carry the particulate material away from the cutter to down-stream operations by means of a high velocity air stream. The high velocity of this air stream causes tearing and breakdown of the polymer crumb, thus creating undesirable fines. Indeed, the primary detriment to prior art cutters is the high pneumatic conveying velocity going through the cutter, which results in shearing and tearing of rubber crumb and generation of fines.
Fines are problematic to the rubber finishing operation from both process and product standpoints. Fines negatively impact the finishing process in that they easily and readily stick to and foul the pneumatic conveying system components, as well as equipment located downstream of the pneumatic conveyor. The fouling problems caused by the presence of fines leads to costly and interruptive down times for equipment cleaning and, indeed, may culminate in the need for component repair and/or replacement.
From a product standpoint, the influence of fines on the finishing process is equally undesirable. Fines which adhere to the hot surfaces of the process equipment will tend to degrade and contaminate the polymer. The extent of fines production in the finishing process thus raises serious concerns of product quality and integrity.
The present invention addresses and overcomes the deficiencies associated with conventional elastomer finishing processes by providing an integrated device for polymer extrudate shape control and conveying. According to the present invention, a novel die means presents a greater surface to volume ratio and shorter diffusion path than conventional extruder drying die means. The ranges of extruder temperature and pressure at or near entry to the die for conventional systems are typically 150.degree. C. to 260.degree. C. and 500-1800 psi for the same volatiles content standards of elastomers. The die means of the present invention is thus uniquely capable of achieving a degree of drying commensurate with conventional devices, but at lower temperatures and pressure, with an accompanying decrease in fines generation and an increase in line capacity.
A further aspect of the invention involves the use of a novel cutter means in association with the die means. This cutter means provides a larger open area for the conveying air stream as compared with conventional cutters. The cutter means of the present invention significantly lowers the local air velocity in the cutter and achieves a significant reduction in crumb breakdown due to tearing. Moreover, the cutter means of the present invention can be operated at a lower RPM than that required for conventional cutters. In particular, the improved cutter means of the present invention is operated at 50-600 RPM and preferably, from 50-250 RPM, as opposed to the manufacturer's recommendations of 1500-2400 RPM for conventional cutters.
Thus, the combined features of the present invention serve to reduce drying temperatures and pressures, to decrease air velocity through the cutter means, to produce larger average crumb size, to significantly diminish the amount of fines generated, and to eliminate fouling of the pneumatic conveying equipment and equipment downstream thereof. Indeed, the size of the crumb produced with the invention closely approaches the theoretically calculated value, thereby providing a mechanism for extrudate shape control.
2. Description of the Prior Art
Rubber drying processes utilizing pressure extruders and die means is known in the prior art. More specifically, the prior art discloses die means for rubber drying including a pair of concentric cylinders mounted normal to the extruder axis and being positionable with respect to each other in order to bring about varying degrees of alignment of die holes provided in each of the cylinders. For example, Porter et al U.S. Pat. No. 3,973,890, which issued on Nov. 8, 1983, and which is commonly owned by the assignee of the present invention, discloses an adjustable die face pelletizer mounted at the output end of a rubber drying extruder. The pelletizer is characterized by die plate means comprising a pair of nested cylindrical die plates operatively arranged for selective positioning with respect to each other. Each of the die plates is provided with a plurality of round die orifices which, by positioning the die plates, may be completely aligned, or may be caused to assume varying degrees of misalignment.
It is also known in the prior art to provide a rotatable cutter within the inner cylinder of a die means, and to direct a fluid such as air into the area of the cutter to carry away cut material. Such is also shown in the aforementioned patent to Porter et al, wherein a cylindrical cutter having radial helically extending blades is mounted concentrically within the inner die plate. A gas or liquid treatment fluid flushes the cut material away from the cutter.
Similarly, U.S. Pat. No. 4,413,971, which issued to Nettleton on Nov. 8, 1983, shows a rubber recovery apparatus comprising a fixed plate containing extrusion orifices, and within which is disposed a rotatable plate provided with extrusion apertures. An extrudate passes through the extrusion apertures, and is cut into pellets by means of rotating cutter blades which project radially from a solid cylindrical body. A compressed gas flows down between the blades to carry the pellets into another chamber. Similar die means and cutter means arrangements are shown in Rossiter et al U.S. Pat. Nos. 4,185,057 and 3,874,835, and Redding et al U.S. Pat. No. 3,578,740.
Various other prior art patents teach die means mounted at the discharge of an extruder, cutter means for cutting material forced through the die means and a conveying medium for transporting cut material from the cutter means. In Craddock et al U.S. Pat. No. 3,645,200, dated Feb. 29, 1972, the cutter means comprises a plurality of hub-mounted rotating blades. The cut material is propelled away by means of the cutter blades.
U.S. Pat. No. 3,574,891 of Apr. 13, 1971 to Bredeson et al teaches a mechanical screw press characterized by a circular die plate having an annular array of circular nozzle orifices associated with a cutter having radially extending knives. Steam is directed into the die plate.
U.S. Pat. No. 3,393,425, which issued on July 23, 1968 to Irving is directed to an apparatus for forming pellets wherein a screw feeds plastic material to the die orifices of a die and a rotary cutter cuts the extruded strands of material into pellets. The cut pellets are conveyed away from the cutter in a zig zag pattern by sprays of cooling fluid.
Burner U.S. Pat. No. 3,382,538 of May 14, 1968, shows a die plate with circular extrusion orifices and a rotatable cutter member positioned adjacent thereto. The cutter includes a series of teeth of spiral configuration, each of which has a knife member mounted thereon.
Several foreign patents are directed to conventional rubber extrusion processes, those being Soviet Union Patent 1242-386-A, Great Britain Patent 2 166 141 A and German Patent 1454749-R.
The prior art fails to teach or suggest a die means which provides a surface to volume ratio and diffusion path to enable degrees of drying commensurate with the conventional extruder drying processes but at lower temperature and pressure. Additionally, the prior art fails to show a cutter means which presents a large open area for conveying air so as to realize significant reductions in air velocity. The need exists, therefore, for a device possessing the latter features, and which achieves a reduction in fines generation and an increase in average crumb size in an elastomer finishing process.